The EFF has been working to crack the code that some color printers put on every printed page. It's been long known that printer manufacturers put these codes in many color printers at the behest of the Secret Service, who's concerned about the potential for counterfeiting. The EFF, however, has revealed just how these codes work (with images).
The images really bring this home. Just think about every document you print containing tracking codes that link it back to the printer in your office or home. The privacy concerns are huge. Imagine that you print a handbill complaining about working conditions and post it in the lunchroom. You might have thought it was anonymous, but in fact, your employer can link it right to the source. I'm not really a big privacy freak, but I don't like this at all.